[Bug c/84815] New: gcc fwrite failed write to stdout in binary mode (Win7)

[albertmcchan at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84815

            Bug ID: 84815
           Summary: gcc fwrite failed write to stdout in binary mode
                    (Win7)
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: albertmcchan at yahoo dot com
  Target Milestone: ---

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    size_t n = 64*1024;                 // code ok if below 64k
    char* buf = malloc(n);
    if (buf == NULL) return 1;
    memset(buf, 'O', n);
    buf[n-1] = 'X';                     // signal for last byte

    setmode(STDOUT_FILENO, _O_BINARY);  // <-- this caused the bug

    n = fwrite(buf, 1, n, stdout);      // fwrite failed, but why ?
    printf("\nfwrite %d bytes\n", n);   // output: fwrite 0 bytes
    free(buf);
    return 0;
}